This invention is related to commonly-assigned U.S. patent application Ser. No. 07/557,257, filed July 24, 1990 of the same inventor for METHOD AND APPARATUS FOR TESTING AN LCD PANEL ARRAY USING A MAGNETIC FIELD SENSOR.
This invention relates to testing of liquid crystal display (LCD) panel arrays, and more particularly to a method and apparatus for testing LCD panel arrays for open circuit and pixel defects by applying test signals to panel shorting bars.
LCD panels typically are formed with a liquid crystal material sandwiched between an active plate and a ground plate. Polarizers, colorizing filters and spacers also are included between the plates. During fabrication, many active plates  panels may be formed on a single glass plate. In each area of the glass plate which is to form an active plate,  panel, drive lines, gate lines and drive elements are formed. Typically, thin-film transistors are used for the drive elements.
Each active panel has an electro-static discharge (ESD) shorting bar at each of the four edges of the active plate. The ESD bar shorts all the drive lines or gate lines which terminate at a respective edge. For an interdigitated panel, drive lines are terminated at two opposing edges while gate lines are terminated at the other two edges. Thus, four shorting bars are included, one per panel edge.
Until scribing and final testing of the LCD panel, the ESD bars remain attached to the panel so as to avoid electro-static charge buildup. Prolonged separation of the panel from the shorting bar or another grounding apparatus may cause the electro-static charge to accumulate and cause damage to the active panel circuitry. Accordingly, a method is needed for testing the LCD panel array with the ESD shorting bars in place.
Referring to FIG. 1, a typical active matrix LCD panel segment 10 is shown consisting of an array of pixels 12. Each pixel 12 is activated by addressing simultaneously an appropriate drive line 14 and gate line 16. A drive element 18 is associated with each pixel 12. The drive lines 14, gate lines 16, pixels 12 and pixel drive elements 18 are deposited on the clear glass xe2x80x9cactivexe2x80x9d plate by a lithographic or similar process. Because of the high pixel densities, the close proximity of the gate lines and drive lines, and the complexity of forming the pixel drive elements, there is a significant probability of defects occurring during the manufacturing process.
Known testing methods for high density LCD panels include contact testing methodologies which require connection to and testing of each individual row/column intersection within the panel array. For such testing, advanced probing technology is necessary to establish reliable contacts among the densely populated pixel elements. A high density LCD array includes 640 by 480 pixel elements per color. A typical test time for such a panel is approximately 2 hours. For a color panel having the three primary colors red green and blue (xe2x80x9cRGB color panelxe2x80x9d), a typical test cycle requires additional connections and requires additional testing time. The time and expense of testing, although necessary, is a limiting factor to the commercial success of large array LCD panels. A faster and more efficient testing method is needed to reduce the testing costs, and thereby reduce the product costs of LCD panels so as to compete with CRT and other display types.
Accordingly, it is desirable to be able to test large arrays easily, without direct individual electrical connection and with connections only as needed.
According to the invention, an LCD panel or the like is tested for open circuit defects and pixel defects after preliminary short circuit testing is complete. According to one aspect of the invention, the panel undergoes open circuit and pixel testing by exposing the panel to test signals at the contacts of each respective shorting bar. The resulting display pattern then is imaged and compared to an expected display pattern to detect panel defects.
According to another aspect of the invention, the resulting display is imaged of a TV camera, line-scan camera or other optical sensing instrument. Such camera or instrument images the display panel and transmits the resulting image signals to a computer system for processing and storage as sensed image data. The computer system compares the sensed image data to expected image data to determine whether there are any differences between the resulting display pattern and the expected display pattern.
As there are only a finite number of test patterns which may be applied to the shorting bars, there are only a finite number of expected display patterns. By having the computer control or monitor the test signal selection, the computer is able to select the appropriate expected display pattern, and thus, select the appropriate expected image data to be compared with the sensed image data.
According to another aspect of the invention, a pixel short circuit defect is detected by applying sequential test cycles of test signals to the shorting bars. During a first cycle, an active signal is applied to the shorting bars which are connected to the gate lines, while another active signal is applied to the shorting bars which are connected to the drive lines. Then, during a second cycle, the active signal which is applied to the drive lines is switched to an inactive signal. If any pixels remain active, then there is a pixel short circuit. Alternatively, the active signal applied to the gate lines may be switched to an inactive state. Any pixels that become inactive result from a pixel short circuit.
The invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings.